What is Cardiac Fibrillation?

Greg Caramenico

Cardiac fibrillation is the erratic and unsynchronized contraction of heart muscle. Since the heartbeat — thus the circulation of blood — depend on synchronized muscular contraction and relaxation, its absence can impair normal cardiac function. When it occurs in the atria, it is called atrial fibrillation and is characterized by the rapid and chaotic electrical activity of cardiac muscle cells. Ventricular fibrillation can rapidly lead to death if not promptly treated, as the ventricles lose their normal rhythm and cease to contract, shutting down circulation.

A defibrillator.
A defibrillator.

A condition caused by many different diseases, cardiac fibrillation occurs when the muscles within the heart contract irregularly. It causes an abnormal heartbeat, also known as an arrhythmia. Afflicting the heart muscle, it can happen to either of the two upper chambers, called the atria, or to the two lower chambers, called the ventricles. While cardiac fibrillation that occurs in the atria can lead to chronic health problems over time and raises the risk of stroke, it is not necessarily an emergency.

Cardiac fibrillation can occur in the atria, at the top of the heart, or the ventricles, at its base.
Cardiac fibrillation can occur in the atria, at the top of the heart, or the ventricles, at its base.

The normal heartbeat pattern — called the sinus rhythm — is directed by electrical currents that travel through cardiac muscle and cause it to contract. Regardless of the number of heartbeats occurring within a minute in a normal heart, the interval between the beats remains the same at any given heart rate. During cardiac fibrillation, electrical signals occur at irregular intervals, which interrupts this cycle with an erratic fast heartbeat. In atrial fibrillation, the electrical current that normally comes only from the heart's pacemaker — the SA node — instead is released from cells all over the atria, preventing the muscle from the having the simultaneous contraction of a normal heartbeat.

Atrial fibrillation is the most common form of cardiac fibrillation. When the atrial cells quiver due to decentralized electrical current, they cannot properly pump blood into the ventricles. Ventricular contractions are slower than the normal atrial rate, and are much slower during the pace of atrial fibrillation. Consequently, if the atrial cells are contracting rapidly, the ventricles will have insufficient time to catch up with them, or fully fill up with blood to pump into systemic circulation. If this condition becomes chronic, there is a risk that the atrial muscle wall will become fibrous and the heartbeat will remain abnormal.

Ventricular fibrillation is very dangerous because blood remains within the heart and is not pumped out. If the normal sinus rhythm is not quickly restored, the chaotic ventricular rhythm will give way to asystole, or flat-lining, causing cardiac death. It is frequently caused by a heart attack or oxygen deprivation in the heart muscles, though less common events such as electrocution and injury can also induce it. Ventricular fibrillation manifests itself in the lack of a pulse and unconsciousness. Patients require immediate medical treatment with an electrical defibrillator to restore a normal cardiac rhythm, and many also need follow-up care.

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